Cascade vendor prefix checks down to a property with no prefix.

Right now we have most of our vendor extensions prefixed with the language or platform in use. So `x-scala-type` describes a custom type for Scala clients, and `x-jvm-type` does the same, but covers all JVM languages. We should also have `x-type`, which would apply to everything. (This is probably not useful for custom types, but would be useful for other properties, like the new `x-{scala,java,jvm}-class-name` extension.)

Referenced in a comment on https://github.com/twilio/guardrail/pull/719

Start by locating the vendor-extension handling and existing checks for x-scala-, x-java-, and x-jvm- properties, using the comment referenced from pull request #719 as context. Confirm the precedence behavior in the relevant tests or add coverage so an unprefixed x-type applies when no more-specific property is present.

Written by the indexing model from the issue text.